1. Field of the Invention
The present invention relates to an optical disk, which is used in a recording and regenerating apparatus beyond NA O. 7, and on which multiple information layers of an optical disk medium is formed, in the optical disk medium used for each of various optical disk apparatuses such as DVD-ROM, DVD-RAM, and DVD-RW.
2. Description of the Related Art
A production method of a conventional optical disk substrate will be described. A plastic substrate on whose surface a concavo-convex pattern is reproduced is completed by injecting plastic substrate material (for example, polycarbonate etc.), which is dissolved at high temperature, in high pressure into a die equipped with a metal stamper, cooling the plastic substrate material, and taking out it. This technique is a present common production technique (injection molding method) of plastic substrates used for CD-Audio, CD-R, CD-ROM, DVD-ROM, DVD-R, DVD-RAM, DVD-RW, MO, etc. Then, a medium is formed by forming a reflective layer or a recording stacked layer on a surface of the concavo-convex pattern by sputtering as a signal-recording surface, and forming a protection layer with a UV cure resin so as to protect the signal-recording surface so that the signal-recording surface may not be damaged.
As means for higher densification, the enlargement of numerical aperture (NA) of a stop down lens and multiple stacking of a recording layer are mentioned. The multi-layer optical recording medium is shown in JP-A-8-297861. This is a multi-layer optical recording medium with large capacity that has two recording layers respectively on a plurality of substrates whose recording media are different from each other, that has the structure where substrates with two-layer recording layers are laminated so that their recording layers may face each other, and that has sufficient record and reproduction characteristics.
Since what is difficult in the case of multi-layer production is to produce uniform intervals (hereafter, thickness of a spacer layer) between layers, it is indispensable to make a substrate, which is in a light-incident side, thinner than 0.6 mm when the NA of a lens is large. In that case, it is difficult to transfer a concavo-convex pattern from a stamper on a substrate in the light-incident side and to form recording stacked layers sequentially on it, like a conventional way. It is because the substrate may be curled due to the stress caused by the formation of the recording stacked layer. Therefore, it is necessary to also transfer the pattern on a surface of the spacer layer and to form the recording stacked layer on it.
As for a production method of a spacer layer, a method of forming the spacer layer with an ultraviolet cure resin by a spin coating method is shown in JP-A-9-73671. A spin coating method is a method of dropping liquid from a nozzle, diffusing the liquid by rotation, and forming a layer. Here, the spacer layer is cured by irradiating the spacer layer with ultraviolet spot light with interlocking with the forming operation of the spacer layer and moving toward a peripheral side from a center of rotation. In addition, another production method of a spacer layer is shown in JP-A-2000-36165 as an example of using a dry photopolymer sheet. A dry photopolymer consists of a polymer, whose functional groups absorb light and react. Here, the production method of a spacer layer is mentioned, the method comprising the steps of placing a resin on a stamper, pressing a sheet with the stamper, transferring a signal, performing UV irradiation to the sheet with the stamper, curing the resin, and stripping the stamper.
As for a production method of a spacer layer, JP-A-9-73671 shows a method of forming the spacer layer with an ultraviolet cure resin by a spin coating method. Nevertheless, for example, when producing a disk with a diameter of 120 mm with this method, the thinner the spacer layer becomes due to shortening of wavelength or achievement of higher NA, the easier the disk is affected by the curvature of a substrate etc., and hence, it is difficult to decrease the dispersion of thickness of the spacer layer. In addition, in an example of using a dry photopolymer sheet in JP-A-2000-36165, it is described that, when a resin layer is heated, the entanglement of a portion where carbon atoms of a polymer lie in a long chain loosens, and hence the resin layer becomes soft to some extent. However, if the extent of this softening is enlarged, thickness becomes uneven when a pattern is transferred with pressing a sheet with a stamper, and when stripping the stamper after transfer as it is, the transferred pattern is collapsed by an adhesion force or the friction in a ramp of the pattern. On the other hand, if the extent of softening is decreased, there is a problem that transfer property is bad since the resin does not go into concavities of the stamper.
Examples of a case where the transfer property is good and a case where the transfer property is poor are shown in FIGS. 16 and 17. Since ROM (read only memory) differs in the geometry of a pattern from RAM (random access memory), each is shown. Since a thermoplastic organic material is softened by heating a sheet when the sheet is pressed against a stamper, the thermoplastic organic material is filled into the details of a stamper pattern, and hence, the concavo-convex pattern of the stamper is faithfully transferred by stripping the stamper after curing. When filling in this case is inadequate, it causes the lack of a pattern, and hence, transfer is unsuccessful.
An object of the present invention is to attain further high densification for producing a spacer layer with good pattern transcription.
The above-described issues are solved by the following methods.
(1) A production method of a spacer layer will be described. This method comprises the steps of forming a first recording stacked layer on a substrate, and heating a sheet with sandwiching the sheet, made by mixing photo-curing organic material with thermoplastic organic material, between the first recording stacked layer and the stamper for pattern transfer to transfer a pattern of the stamper to the sheet. The stamper is stripped after pattern transfer, and after that, light is radiated on the sheet. When photo-curing organic material and thermoplastic organic material softened by heating are mixed, the fundamental geometry, i.e., thickness, of a resin layer is maintained by the photo-curing organic material when the resin layer is heated, and the thermoplastic organic material distributed inside the resin layer in a granular or braided shape is remarkably softened, and hence, it becomes easy to transfer the concavo-convex pattern of the stamper by surface deformation. In photo-curing organic material, there are cationic polymerization type material that gradually performs curing reaction when light is radiated, and another type material that is cured immediately after light is radiated. When a reaction rate is slow, light irradiating is performed before pattern transfer to a spacer layer as a pattern for a second layer, and when a reaction rate is fast, light irradiating is performed after pattern transfer. Photoresistive organic material comprises, for example, a resin and a cationic initiator, and as examples of this type of resins, a cycloaliphatic epoxy resin, a bisphenol A type epoxy resin, a bisphenol F type epoxy resin, a phenol novolak type epoxy resin, a vinyl ether compound, an acrylic resin, etc. are mentioned. Photopolymerization initiators are, for example, benzoin, benzoin butyl ether, benzoin isopropyl ether, benzophenone, benzaldehyde, benzyl, benzoin ethyl ether, benzyl dimethyl ketal, xanthone, thio-xanthone, azobisisobutyronitril, benzoyl peroxide, a benzophenone/amine system. Mixing ratio is 0.01 to 10 wt % of cationic initiator to each resin. And photopolymerization initiators (photochemical sensitizers) commonly known are mentioned, whose mixing ratio is 0.1 to 5% to each resin. As thermoplastic resins, for example, a phenoxy resin, a polyvinyl acetate resin, a vinyl butyral resin, an acrylic resin, a polyvinyl chloride, a vinyl chloride copolymer, polyvinyl alcohol, a cyano acrylic ester system, a nitrocellulose system, polyyne butylene, polyvinyl ether, acrylic ester, polyvinyl pyrrolidone, polyolefine, polypropylene, nylon, polytetrafluoroethylene, etc. are mentioned. It is still more preferable if a thermoplastic resin is also polymerized by light irradiating and a molecular weight increases.
(2) In addition, another production method of a spacer layer will be described. This method comprises the steps of forming a first recording stacked layer on a substrate, and heating a sheet with sandwiching the sheet, made by mixing thermosetting organic material with thermoplastic organic material, between the first recording stacked layer and the stamper for pattern transfer to transfer a pattern of the stamper to the sheet. The stamper is stripped after pattern transfer, and after that, the sheet is heated. When thermoplastic organic material of excellent property in mass production by conventional injection molding is used for a spacer layer of a multi-layer recording medium, the material may be changed with the heat of a recording laser beam. In order to prevent the change, it is good to mix thermosetting organic material that performs bridge-forming reaction and is cured with heat. This is because there is no heat deformation after complete curing. In thermosetting organic material, there are cationic polymerization type material that gradually performs bridge-forming and cures when heat is applied, and material is cured immediately after heat is applied. A resin similar to a thermoplastic resin mentioned in item (1) can be used. As for thermosetting organic material, phenol resin, resorcinol resin, phenol resorcinol resin, urea resin, polyester, epoxy resin, polyisocyanate, a silicone resin, acrylic resin, acrylic acid diester resin, melamine resin, etc. are mentioned. In addition, mixtures of respective polyvinyl acetal, polyvinyl butyral, polyvinyl formal, nylon, neoprene resin, etc. to respective phenolic resins, and mixtures of respective cationic initiators to these respective resins, whose mixing ratio is 0.01 to 10 wt % to each resin, are mentioned.
(3) Furthermore, still another production method of a spacer layer will be described. The method comprises the steps of forming a first recording stacked layer on a substrate, placing thereon a sheet that is made by mixing photo-curing organic material and thermosoftening-then-setting organic material whose molecular weights are different by 5 or more times, and more preferably, 50 to 100 times, radiating light, and transferring a pattern of a stamper on a sheet with sandwiching the sheet between the substrate and stamper and heating the sheet. Subsequent steps are steps of stripping the stamper after pattern transfer, and radiating light on the sheet thereafter. Thermosoftening-then-setting organic material is softened at a first temperature and self cured after the softening, and the concept of the thermosetting organic material described in item (2) includes the thermosoftening-then-setting organic material. An epoxy resin is mentioned as a typical example. Although the thermosoftening-then-setting resin is excellent in thermal stability after curing, since high reaction temperature becomes a process problem, it is also preferable to enhance a reaction efficiency by adding a photopolymerizing component. As shown in FIG. 12, when small molecules are made to enter into gaps between large molecules, it is possible to maintain fundamental geometry, that is, thickness with large molecules and to have further excellent film uniformity because small molecules are dispersed in a granular or brained shape. For example, when the molecular weight of the thermosoftening and thermosetting material is large, fundamental thickness is maintained although the material becomes soft with heat, and photo-curing organic material with the small molecular weight distributed in the gaps perform curing reaction by light irradiating to be firmly solidified. When there is no difference between molecular weights, the thickness uniformity or transcription becomes bad. However, if one of the two molecular weights is too large, dispersion becomes nonuniform and the sheet is only partially softened even if heat is applied. Thermosetting organic material similar to that described in item (2) can be used as the thermosoftening and thermosetting organic material, and photo-curing organic material the same as that described in item (1) can be used as the photo-curing organic material.
(4) In addition, the sheet material in item (1) is characterized in that, when a softening degree of thermoplastic organic material is larger than that of photo-curing organic material, it is more preferable that a content of thermoplastic organic material in a side far from the substrate is larger than that in a side near the substrate. It is desirable that, when a pattern for the second layer is produced for a multi-layer recording medium, a resin in a stamper side is more thermosoftening, because more faithful pattern transfer from a stamper such as a Ni stamper becomes possible.
(5) In addition, the sheet material in item (1) is characterized in that, when a softening degree of photo-curing organic material is larger than that of thermoplastic organic material, a content of thermoplastic organic material in a side near the substrate is larger than that in a side far from the substrate. It is desirable that, when a pattern for the second layer is produced for a multi-layer recording medium, a resin in a stamper side is more thermosoftening because more faithful pattern transfer from a stamper such as a Ni stamper becomes possible.
(6) This is characterized in that each spacer layer mentioned in items (1), (2), and (3) has light transmission of 90% or more in wavelengths of 390 to 415 nm. This is because, when record and reproduction are performed with radiating a laser beam, both of the spacer layer and a cover layer become optical transmission layers. In addition, also in the case that the above-described sheet comprises a resin layer and a transparent base sheet, it is desirable that the light transmission of the resin and transparent base is high at a wavelength of 405 nm. The desirable material of the transparent base is polycarbonate or polyolefine. Since the beam is absorbed in both ways of incidence and reflection when a laser beam is incident into the spacer layer, high transmission is desirable. If the transmission is 89% or less, influences such as a signal error rate increase caused by a small amounts of reflected light arise.
In addition, in the case that the sheet comprises a resin and a transparent base, it is desirable that the square of the difference between a refraction indice of the resin and that of the transparent base sheet is {fraction (1/150)} or less. Since the sheet is used for a transparency layer of a multi-layer recording medium, a laser beam goes back and forth the first recording stacked layer and second recording stacked layer. Hence, it is desirable that the refraction indices of the above-described resin and the above-described transparent base are about the same. This is because optical interference exerts a bad influence.
Owing to these, it was succeeded to experimentally produce a recording medium for a super-large capacity optical disk digital video recorder with a capacity of 40 GB or more and a recording medium for an optical disk digital camcorder with a capacity of 4 GB or more.
(7) Case of Light Irradiating:
An apparatus which forms a second recording stacked layer sequentially on the first recording stacked layer will be described by using FIG. 2, and process steps thereof will be described by using FIG. 8. This apparatus has the configuration that a stripping part 17 of a stamper is provided between a heating and pressing part 21 for transferring a pattern on a sheet and a light irradiating part 16 that cures the sheet. This part presses the sheet with melting the sheet by heating. Then, heat sources such as a roller and a heater that are heated at high temperature are mentioned as heating means, and a roller, pneumatics, and a tool with stamp geometry are mentioned as pressure means. In addition, as for light irradiating means, when wavelengths to which an optical curing agent reacts are within UV, UV light sources to be used are a metal halide lamp, a low-pressure mercury lamp, and a high-pressure mercury vapor lamp. A holder of the stamper stripping part has a mechanism raising the stamper with bending the stamper, and is equipped with a magnet chuck 9 for stripping. In addition, besides a colored stamper like a Ni stamper, the stamper can be made of polyolefine and the like that are transparent. On the other hand, JP-A-2000-36165 describes an example that a sheet is softened by heat, UV emission is performed with the sheet being pressed with the stamper, and the stamper is removed thereafter. However, in the colored stamper such as the Ni stamper, since UV light is interrupted by the stamper with the stamper being pressed, the UV light cannot be adequately radiated on the sheet. Here, in the method of the present invention, since light irradiating is performed after the stamper being stripped, the stamper does not interrupt the UV light. In addition, according to JP-A-2000-36165, since the stamper is removed after radiating UV light, the stamper is removed after a sheet is completely solidified, and hence, not only it is difficult to remove the stamper with maintaining transcription, but also stress causes a crack. On the other hand, according to the present invention, since the stamper is removed before a sheet is completely cured, not only it is possible to prevent a crack from arising due to stress, but also it is possible to obtain good transcription.
(8) Case of Heating:
An apparatus which forms a second recording stacked layer sequentially on the first recording stacked layer will be described by using FIG. 3, and process steps thereof will be described by using FIG. 9. This apparatus has the configuration that a stripping part 17 of a stamper is provided between a heating and pressing part 21 for transferring a pattern on a sheet and a heating part 18 that cures the sheet. At the same time when becoming soft by being heated, a thermoplastic resin is filled up over the details of a stamper pattern by being pressed. Furthermore, the sheet becomes cured as temperature falls, and simultaneously, curing reaction by heat also occurs. Since the stamper is easily removed before the sheet is completely cured, it is possible to prevent a crack from arising due to stress. In addition, since the pattern deformation does not arise at the time of stripping the stamper, transfer is excellent. Furthermore, after a stamper is stripped, the sheet is heated by the heating part 18 and is completely cured.
(9) Case of Optical Re-emission After Heating After Light Irradiating:
An apparatus which forms a second recording stacked layer sequentially on the first recording stacked layer will be described by using FIG. 4, and process steps thereof will be described by using FIG. 10. The apparatus comprises a light irradiating part 19 for facilitating a thermal reaction, a heating and pressing part 21 for transferring a pattern on the sheet, a stripping part 17 of a stamper, and a light irradiating part 20 that finally cures the sheet. It is made the thermal reaction to easily arise by first light irradiating, a reaction gradually progresses by temperature rising by the heating part, and a thermosoftening and thermosetting resin which becomes soft is filled over the details of the stamper pattern by the pressing part. Furthermore, the sheet which becomes cured by temperature rising is stripped before being completely cured, and is further completely cured thereafter by light irradiating.